The beta 17-hydroxy-C19-steroid dehydrogenases of guinea pig kidney, dog prostate and human prostate (carcinoma, benign hypertrophy) will be purified and their chemical and physical properties and enzymatic activity including coenzyme requirements compared. The tissues will be homogenized, the subcellular fraction containing the enzyme activity solubilized if in the microsomes, treated sequentially by (NH4) sub 2 SO4 precipitation, Sephadex filtration, DEAE-cellulose chromatography and other protein methods to obtain a crystalline product. Acrylamide gel electrophoresis will be used to ascertain purity and/or heterogeneity and molecular weight. The enzymes will be analysed for amino acid composition and the sulfhydryl and end groups determined. The enzymes will be hydrolysed by enzymes and chemical means to determine the active portion of the molecule and the part associated with its specific coenzyme - NAD or NADP.